In recent years, inkjet recording devices have come to be used more than gravure printing type or flexo printing type devices, which require prepress process, because inkjet recording devices allows easy and inexpensive image forming.
In the field of image recording on commodities or packing materials for commodities by the use of such an inkjet recording device, materials such as resins or metals, which do not absorb ink, are often used for commodities or packing of commodities. Photocurable ink is used to fix ink on a recording medium which does not absorb ink. After jetting photocurable ink onto a recording medium, light such as UV light is projected onto the ink to fix the ink by curing.
In this field, images including characters, symbols, and pictures are recorded with a background color such as white on a transparent packing material, for example. In case of printing a background on a transparent packing material in such a manner, front recording or back recording is performed. In front recording, background ink is first jetted onto the front surface of a recording medium in a solid state, and then an image is recorded on the background ink. In back recording, an image is recorded first on the back surface of a recording medium, and then background ink is jetted in a solid state on the image. Further, in recording an image on a color recording medium, sometimes, color ink is jetted onto white ink having been jetted in the shape of the image so that the image is recorded to improve the coloring of the image, or, transparent ink is jetted onto the surface of a recorded image to improve smoothness and gloss tone.
As an inkjet recording apparatus for image recording by the use of inks in various colors mostly for image recording and an auxiliary recording ink such as a background ink or a transparent ink that enhances image recording, there are known inkjet recording apparatuses (see Patent Document-1, for example) as follows. That is, there is known an inkjet recording apparatus which includes recording heads with nozzles for jetting ink, reciprocally moves the recording heads in the main scanning direction while jetting ink from the recording heads onto a recording medium, and conveys the recording medium in the sub scanning direction orthogonal to the main scanning direction, wherein an auxiliary recording head for jetting auxiliary recording ink is disposed on both sides, in the main scanning direction, of the image recording heads for jetting image recording inks in respective colors.
For example, in performing front recording by such an inkjet recording apparatus, the apparatus records a background by jetting ink in a solid state from an auxiliary recording head positioned on the upstream side of image recording heads while the recording heads are moving in both the outward direction and the return direction, and by jetting ink from the image recording heads based on image data to record an image. In performing back recording, the apparatus does not jet ink from an auxiliary recording head positioned on the upstream side of the image recording heads while the recording heads are moving in the outward direction nor the return direction, but jets ink from the image recording heads to record an image based on image data and jets ink in a solid state from the auxiliary recording head positioned on the downstream side of the image recording heads to record a background.
Further, there is also known an inkjet recording apparatus (see Patent Document 1, for example) which includes auxiliary recording heads disposed on both the upstream side and the downstream side, in the sub scanning direction, of image recording heads for respective colors.
For example, in performing front recording by such an inkjet recording apparatus, the apparatus jets ink in a solid state from an auxiliary recording head positioned on the upstream side of image recording heads in the sub scanning direction to record a background, and after the recording medium on which the background has been recorded is conveyed in the sub scanning direction, the apparatus jets ink from the image recording heads onto the part where the background has been recorded, so as to record an image. In performing back recording, the apparatus does not jet ink from an auxiliary recording head positioned on the upstream side of the image recording heads in the sub scanning direction, but jets ink from the recording heads to record an image, and after the recording medium on which the image has been recorded is conveyed in the sub scanning direction, the apparatus jets ink in a solid state, from an auxiliary recording head positioned on the downstream side of the image recording heads, on the part where the image has been recorded, so as to record a background.
[Patent Document 1] TOKKAI No. 2003-285422
However, in a known inkjet recording apparatus, an auxiliary recording head is arranged on each side of image recording heads in the main scanning direction, or at least on one of the upstream and the downstream sides in the sub scanning direction, which causes a problem of a larger size of a carriage having the recording heads and also a larger size of the entire apparatus, and a high manufacturing cost of the apparatus. Further, if an auxiliary recording head is provided at least on one of the upstream side and downstream side of image recording heads in the sub scanning direction, the area of a platen supporting a recording medium becomes larger to cause a problem of degrading image quality due to drop in conveyance accuracy caused by friction and absorption force between the platen and the recording medium.
Further, particularly with an inkjet recording apparatus for recording an image using photocurable ink, in order to prevent mixture of inks jetted spuerimposedly and blurring of ink, it is necessary to project light onto ink having been jetted so that the ink is fixed on a recording medium and thereafter jet ink newly.
Therefore, in the case where auxiliary recording heads are provided respectively on both sides of image recording heads in the main scanning direction, light emitters for projecting light onto ink having landed on a recording medium are needed to be provided not only on both the side portions, in the main scanning direction, of a head unit having the image recording heads and the auxiliary recording heads, but also between an image recording head and an auxiliary recording head, causing problems of a larger size of the carriage, degradation of image quality due to heat created by the light emitters, and a larger power consumption.
Further, in the case where an auxiliary recording head is provided at least on one of the upstream side and the down stream side of image recording heads in the sub scanning direction, in order to simplify the structure of a light emitter and to project light onto ink jetted from both the image recording heads and the auxiliary recording head, the light emitter is arranged from the upstream side to the downstream side of the image recording heads and the auxiliary recording head in the sub scanning direction, which causes the problems of a large size of the light emitter, degradation of image quality due to heat generation, and large power consumption.